Multihull Structure Thoughts

Thanks Oldmulti for the call out to Igor and Beto. They are incredible sailors and very nice/ humble guys to boot. Loved your synopsis on Beto's (and Igor for a lot of it) sailing resume! For some reason most of these achievements have gone under the radar in the US.... but think these guys need more credit for how amazing sailors they are.

Some more info on the boat-- we built her here at the Turn Point Design shop in 2016. I cannot help but hold my breath until they make it out the other side safely. It originally shocked me when I heard what they wanted to do with my boat... but there are some details in its construction and design that make it different than most all out racing catamarans. The construction was prepreg carbon with nomex cores like many high end racing cats, but the laminate thickness and core thickness is greater than an inshore racing catamaran would be. Also, bellow the waterline is solid reinforced carbon laminate which is quite robust for a small cat. Her bare hull weight without hardware is 450lbs... which we could have brought down to less than 375 lbs if we wanted to make her as an in shore racer. There is 30" of collision foam in the bows and 6 water tight compartments. The resin system was a carbon nano-tube toughened epoxy made for the military... we got a truckload of this carbon prepreg from a local aerospace supplier because they were moving . This toughened resin system gives the laminate better impact/ damage resistance than standard epoxy layups.

You are right about the risk of sailing a small, light, high power cat in open water. It is the kind of boat that will capsize quite rapidly in a gust if you are not paying close attention. In the R2AK it took the helmsman's 100% attention while sailing to keep her moving at speed with a crew at the ready. This can lead to crew fatigue after a couple days underway which will be something Igor and Beto are very experienced at addressing. I cant think of any other team of sailors that have the depth of ocean experience in small cats that Igor and Beto do. There are some advantages of such a small cat for the NW passage. The first being that it is possible to beach the boat and even pull it over the ice for short distances. Another advantage is that the boat is configured to be self rescuable--- the mast is sealed to prevent turtling and the mast cants to help push the boat right side up. A third very good reason for a lightweight cat is that it is possible to human power the boat effectively. Their boat is fitted with a pedal drive system that allows them to propel the boat at 3-4 knots. They have used this quite a bit so far in their first 800 miles.

You can follow them on their tracker at rotapolar.com.br

 

Matt Tucker has had an interesting life around boats. He has lived on a home built (by his parents) 48 foot monohull, sailed to the Antarctic, raced and cruised on many other types of boats. He currently runs a boat building business ( https://www.evergreenboats.com.au/ ) building updated classic designs. So, it’s only logical that he should build a 27 foot experimental proa so he can sort out the design concept before he builds his larger ocean racing proa.

There are few details and pictures of the proa named “UFO” but I will tell what I know. Matt said “UFO is a prototype with the idea being to test a range of ideas and systems before building a bigger offshore racing Proa. If you think about the overall weight of the proa, it’s quite light, so you use that extra weight advantage to add extra length to the main hull. That gives you a longer, skinnier hull, which is what drew me to this project. You don’t need to build two or three of the same hulls to get the same effect, so the weight saving is massive.”

The length over all is 27 foot. The rig is a Nacra mast with F18 sails. The rig and sail controls are all brought down to a circular 32 mm Ronstan I beam track. This allows the entire rig and sail controls to be able to rotate through 360 degrees for shunting. Within this, there is a rotating wing mast, with the angle being set relative to the boom, like some beach cats. It looks complex but is actually very simple to sail with according to Matt.

The only standard part of the proa is the rig, the construction was basically just foam, fiberglass and plywood, with a bit of carbon as those materials were suitable for the different jobs. Nothing overly high tech. The windward beams are aluminium tubes and the leeward beams are carbon fiber.

The balance is the hardest thing to get right on a proa according to Matt. EG being able to shunt without getting stuck in irons. It’s something that every proa person has trouble with. That’s part of the reason the rudders are out too leeward, to offset the ama to windward (and maybe have foils one day). In terms of the control and the sailing feel of the boat, it feels exactly as wanted. Well balanced, easily driven.

Sorry about the limited jpegs but I think it gives the idea.

 

This will be about the Paper Tiger catamaran. Today will be about the original design and build, tomorrow will be about the same design but the variations of build they have allowed. The Paper Tiger was designed in 1980 by Ron Given, a talented NZ designer of multihull sail and power catamarans. The Paper Tiger is 14 x 7 foot will a platform weight of 110 lbs and a rigged weight of 160 lbs. The 22.3 foot mast carries a mainsail of 100 square foot. The hull length to beam is 12.7 to 1. The draft of the hull based daggerboards is 3 foot. The kickup rudders are transom based.

Performance claims vary but there are claims of 18 knots peak speed although Paper Tiger sailors think the cat is at it best in 8 to 12 knot winds where it is very competitive with other similar size cats.

Now we get to the build. The original design was in ply and timber with aluminium tube cross beams. The ply hulls are 4 mm with taped chines and timber gunnels 25 x 30 mm maximum (outwales). The plywood should be as light as possible EG a sheet of Gaboon 2440mm x 1220mm x 4mm can weigh between 4.5 and 6 kgs per sheet. The cross beams are a maximum of 50 x 50 mm x 3 mm square tubes. There are up to 10 ply bulkheads in the hulls. Now a little explanation. Paper Tiger rules clearly specify the shape of the hull but allow you to build the hull in any materials you want. The only requirements are the size of the gunnels and the crossbeams. The minimum weight of the Paper Tiger hull and crossbeam must be 110 lbs or more. Most boats are near minimum weight.

There are over 14,000 Paper Tigers globally both professionally and home built. Many people have a lot of fun with these cats. The jpegs give the idea and the extra is a PDF doco of the class rules. Tomorrow we will talk about the alternate material builds of Paper Tigers.

 

Richard Woods Saturn is multi chine. I guess depending on your perspective it's either the best or worst of all worlds. Everything is a compromise. The chines on saturn are narrow enough to probably not need much reinforcement. If built in foam no stringers are needed. 29' similar weight and build time/cost as gypsy can be built demountable 10:1 so should be fairly fast. Less headroom than gypsy.

I mention it as I overlooked the design for years but once I had a proper look I bought the plans. I think it's superb.

 

The second part on Paper Tiger catamarans. As I said yesterday the 14 x 7 foot catamaran has a minimum hull and crossbeam weight of 110 lbs. If it is carefully built in ply timber with aluminum beams it will hit this weight. But several production builders wanted to productionise the cat and building a female mould for the hulls could speed up the building significantly. So here were quite a few options in the hull build process.

We will start with a plywood shell but internal framing was thick polyurethane or Styrofoam frames and floors to stiffen the hulls. The only areas of plywood or timber are in the daggerboard case or under the cross beams. These boats started with ply decks but more than one boat replaced the wood decks with foam glass or foam carbon fibre decks. The logic is “lower the centre of gravity” of the hulls.

Next came the first solid fiberglass hulls which had a weight problem or a stiffness problem. As everything other than the hulls are pretty standard aluminum (witin limitations) the hulls boards and rudders were the only parts that could be “lightened” to meet minimum weights.

Then came the first of the foam e-glass hulls. The 4 or 6 mm Divinycell foam cores are covered by 2 or 3 layers of 100 gsm e-glass cloth. Additional reinforcement went under crossbeam points and around the daggerboard cases. The rudder “cases” by now had been simplified to aluminium tubes which bent back to form tillers. The foils were changing from wood to foam glass foils. These cats were stiffer and could get down to minimum weight.

Finally, the production Paper Tiger hull/deck assemblies are made from vacuum bagged fibreglass/foam sandwich. The standard PT (from one manufacturer) hull/deck incorporates fibreglass/vinylester laminates on Klegecell foam with carbon fibre and Kevlar reinforcing at the beam attachment points. The surface finishes are Aquaguard gel coat. Internally, the hulls are fitted with substantial foam sandwich ring frames at the beams, a half ring frame at Station 2 (forestay location) and a fully sealed bulkhead either side of the centrecase creating a very strong centrecase and also dividing the hull into two buoyancy chambers. The centrecase is designed to take the moulded high aspect A-Class centreboards that we currently make from the McKenzie Composites mould (as used by the current A-Class World Champion) these centreboards are made from carbon/vinylester/foam core with white gel coat finish.

Why do people choose to use foam glass and carbon to get a boat that can be built in plywood for about the same weight? 2 reasons. First it is easier to produce a foam glass boat from a mould. But more importantly for high performance sailors, the foam glass boats are stiffer and initially hold there shape better. Also, the weight can be centralised and lower the centre of gravity. For the average sailor they may not notice the difference but race winners can “feel” the difference especially in a seaway.

The jpegs are of a ply with internal foam supports. Production foam glass boats look no different externally but I cannot find any internal shoots. These are fun boats.

 

The space and performance of multihulls is always promoted as “20 knots possible”. I can assure you very few multihulls can sail at 20 knots constantly, they may down a wave in strong winds peak at 20 knots but the majority of cruising boats will not see 20 knots. A series of items were on the Farrier tri group in answer to a person who was thinking of buying a F 27 tri. He wanted to know the real space and performance of the F 27 and F 31 tri’s.

Martyn Adams who has owned a F 27, F 31 and a Mahe 36 foot catamaran provided a good answer.

“I am afraid that you are asking questions that don’t have a single answer. Others have given wise council. To your questions…

1- Yes… but it has nothing to do with the 4’ difference (F 27 vs F31). Available useable volume is the salient criteria I would use if I were you. I’m 6’3 and 250. My wife is 5’11 and 170. We are in our mid 70s. From 2000 to date we’ve owned an F27 for 8 years and around 16,000 sailing hours, a pair of 31s for 7 years and another few thousand hours and a 36’ cat since 2015 and a couple thousand hours more. Volume and carrying capacity for us is as follows: F27…like backpacking so be very critical about what you bring, F9/F31…like a small camping trailer or large truck camper so you have more room than your backpack but it’s not a house, the Mahe cat (36 foot full bridgedeck cat)…it’s a small house.

2- Let’s look at maximum performance for the 3 boats above. Others may be more or less but this is roughly what we found.

F27… In light air about 50-60% True Wind Speed (TWS). Close hauled in under 20 knots TWS about 7.5 knots. I’ve seen 10 knots but doubt that I was actually close hauled.

Beam reach (90* AWA @ 20TWS your choice of sails) less than 18 knots. Deep reach (>120*AWA) around 60% TWS, maybe less.

F31… same conditions… In light air about 50-60% TWS. Close hauled about 10 knots. (Seen 15 knots but it was sailing apparent wind and VMG sucked).

Beam reach about 20 knots (max was 24 knots and change but with greater wind speed). Deep reach around 60% TWS.

Cat…same conditions… In light air about 40-50% TWS. Close hauled 7.5 knots. But sailing 15* farther off the wind.

Beam reach about 10 knots. Deep reaching about 7.5 knots.

Cruising the F-boats, those were target speeds but off the wind I was 5-10 knots slower to be conservative. Cruising the cat…same speeds.

3- When we first got the F27, I imagined that I would be doing 18-20 knots all the time. Now I figured 10 knots averages for the F-boats and 7.5 knots for the 36 foot cat.

I know that I have not answered your questions directly but hopefully this gives you a little better perspective. None of the F-boats are pickup trucks or SUVs. They are great little sports runabouts designed to be kept light. Cruising spans a vast genre. What some consider cruising others would not even consider day sailing and that’s fine. However, if you consider cruising to encompass 4+ adults and a plethora of other gear and you want 15+kts average cruise speed and under 60k, I doubt that it will be an F- boat.”

Another entry about a Gemini 105C. “I chartered and sailed one out of Anacortes for a week of sailing around the San Juan and Gulf Islands. It was an enjoyable week. However, that boat was sooo slow. Seems to me that we rarely got boat speed up to 50% of TWS. I was very surprised and disappointed. My father went from a Farr 1020 to a Gemini 105 and claimed that it was like sailing a dock! Tony Smith liked to claim that it would sail upwind with a J35, but I am not inclined to agree. It was a comfortable and spacious cruiser, an entirely different animal than our F-9AX.”

These are real experiences and I think provide a good insite into a cruiser racer perspective. A pure racer may get higher speeds but you will not have any cruising gear inside. The jpegs are of the boats mentioned.

 

The Skater 308 Outboard Pleasure is a power catamaran that is 30.5 x 9.33 foot and weighs 3000lbs. They are twin outboard powered. The engines of choice are either 2 Mercury 300 S or R’s (about 300 HP each) or 2 Mercury 400 R (400 to 450 HP each). Your choice of engines means either 100 MPH or 121 MPH (195 KMH). The 2 fuel tanks are 100 gallons each. The range with the 400 R engines is 500 miles in cruising mode, about 50 knots. The boat with engines and some fuel weigh 5000 lbs. The hulls have an 18 degree half hull V in each hull. PS the most engine power I have seen in a non race 308 was two 710 HP stern drives, I do not know what the top speed was.

Why the interest? The build of these boats to handle the speed is good. The fastest Skater model is a full power 468 which has been recorded at 172 MPH (290 kmh). So, what is the build of a 308 OP. The entire hull, deck and subcomponents are built using S glass, Kevlar, carbon, balsa sandwich construction with epoxy resin. There are 5 structural bulkheads with all fittings and joints through bolted with stainless steel bolts. The fuel tanks have an Ethanol resistant liner with 75 mm Aircraft fuel fills, fuel pickups, fuel senders and vents.

As you will see in the jpegs these hulls have many steps throughout the design for performance an to add rigidity. There are longitudinal stringers on the floor of the deep V and the cross beam bulkheads are full depth. Balsa is probably used to handle the impact loads the cat suffers at speed. The fact that S-Glass and Kevlar is used in the construction as well as carbon fiber should give a clue that in some instances S-Glass or Kevlar can handle impact loads better than carbon fiber but higher grade carbon fibre is ultimately stronger than S-Glass. I cannot confirm but it looks like S-Glass outside the balsa and carbon fibre inside which would make sense. PS Low grade carbon fibre and S-Glass are about the same tensile strength, but S-Glass elongates more than carbon. High Grade carbon fibre has higher tensile strength but is less flexible. Either do one or the other in a layup do not mix the 2 in a single layup, carbon has the least stretch and will take all the load before the glass starts to load up. You have to be smart to integrate Kevlar into a mix like this.

The jpegs give the idea. Interesting boats that cost $200,000 plus tricked up and only cost you $US800 of fuel for a fast 3 hours of fun.

 

Twistmate is a German produced day sailing pacific proa that is 16.3 x 11.4 foot with a weight of 170 lbs. The mast is a 21.5 foot high carbon fibre and carries a 130 square foot Mylar mainsail. The rig tilts toward the outrigger, letting the wind blow over it. A strong rubber- chord attached to the mast- foot gets stretched. This rig lifts and turns upwind automatically when the boat is in the correct position to the wind. The mast base sits on the wire between the 2 cross beam and rudder mounts mouldings. The rudder daggerboard system allows them to kickup if hit or kickup the rudder not needed for steering. The proa can be assembled or disassembled in 30 minutes by one person. He components can be car topped. The crew can be 1 or 2 people.

Due to the beam, light weight and power available because of the beam of the 16 foot proa the Twistmate performs quite well. As you can see in one jpeg.

The hulls are built from polyester e-glass with aluminium crossbeams and a mesh trampoline.

The jpegs are limited but they give the idea.

 

Malcom Smith is the designer of some interesting proa’s EG Mi 6. He has done an experimental design of a new sail concept with a cat or proa. The ArcSail concept has been around for a while but Malcom has rethought it and wanted to apply it to multihulls. Malcolm built 2 900 mm test models to explore the concept in 2012.

The ArcSail Slewing Catamaran features a tail which provides both horizontal stability and keeps the sail correctly aligned with the apparent wind. It should be inherently self steering and dynamically stable. The way it is configured it can’t sail very far off the apparent wind, a beam reach is about all it can manage (unless you sail it backwards, which is an option). Sailing it downwind requires that it sail very fast in order to generate large apparent wind angles. This is feasible as long as it can be built very lightly. It is not a boat that can carry a large payload, so it’s not particularly useful unless you simply want to go very fast. The most difficult part of designing the ArcSail itself is to design a lightweight structure. The slewing catamaran version features a solid thick aerofoil which would be expensive to build lightly.

The ArcSail Proa is a much more practical boat. A proa hull is easily driven and doesn’t need a huge sail plan to get up a reasonable turn of speed. The proa version is not designed as a high speed craft, more as an interesting and easy to sail cruising vessel. Because there is no heeling moment, the proa can be sailed equally well on either tack and it can also sail in either direction by shunting. It’s a boat that is inherently safe and manoeuvrable.

The proa version of the ArcSail is, I think, much more interesting. It features a simple flat double edged aerofoil. It’s not an efficient section but one of the advantages of a ring wing is that it is very resistant to stall, so even a flat aerofoil should be capable of generating a reasonable amount of lift if a large angle of attack can be achieved. The flat aerofoil can be constructed by stretching a fabric sail between flexible glass fibre or carbon fibre tubes or rods on the leading and trailing edges. The flexible rods are held in the circular arc shape by using wire stays in the same manner as spokes on a bicycle wheel. Hence the fabric version of the sail will be lightweight and can be erected and stowed similarly to a flexible rod tent. The sail is mounted on a low windage cross beam that rotates on a short vertical shaft.

The jpegs are of the models he built. There has been little follow up information on these models or of anyone else who may have tried idea.

 

Reefing would be interesting!

 

Sanity will return tomorrow but in the mean time we will discuss the latest in conceptual design by Steve Kozloff from California. He is the son of Alex Kozloff who was a pioneer of multihulls etc. The “Trident 100” is 328 x 127 foot (yes, the numbers are correct) with a displacement of 670 tons. The rig is “rigid” sails attached to folding wing masts, the air draft is 228 foot that can be reduced to 139 foot. The rigs can rotate through 360 degrees. The length to beam on the main hull is 22 to 1. The draft is 18 foot. As expected, everything is computer controlled and mainly automated.

At this size the 5,100 square feet accommodation on the 6 decks is anything you want. The basic design has accommodation that includes a 680 square foot an owner's suite, VIP suites, 10 guest cabins and cabins for up to 20 crew. There are many onboard lounging and entertainment options, but the large piano bar and saloon on the upper deck act as the hub of the vessel, from which a network of walkways will lead you to other sections. But you can also choose to play with in the 2 swimming pools, 2 spas, 2 helicopters and associated pads, seaplane, multiple power or sail boats and the almost obligatory submarine.

These vessels are to be constructed with ice resistance steel hulls and aluminium upper decks. The first vessel is estimated to cost $400 million depending on the owners’ requirements. Gold plated taps and Renoir paintings do cost a bit.

In terms of speed, the estimate is the Trident can hit 12 knots (14mph) under sail power. She is also fitted with a hybrid (diesel-electric) system that enables a top speed of 17 knots. She can cover 6,700 nautical miles when cruising at 14 knots (16mph) on diesel alone.

As I said reality again tomorrow but the jpegs give the CAD concept and hopefully someone’s dream. If the Trident 100 is to expensive, you could look at the last jpeg which is the Trident M11 sketch which is only 170 foot long, maybe that will fit into your budget.

 

Guzzi3 likes the Woods designed Saturn design and is thinking of building one. Saturn is a pod cat that can be built in parts with 2 hulls a pod cabin and crossbeams. This allows separate component builds which can be assembled or disassembled as required. The Saturn is a more performance orientated version of the Gypsy and is not intended to be a deep ocean sailor. It can sail out of sight of land but it is not a large load carrier. A Saturn is 28.9 x 18.5 foot that weighs 4,000 lbs and displaces 5,400 lbs. the fixed 39 foot mast aluminium mast carries 290 square foot mainsail and a 158 square foot genoa. The length to beam on the hulls is 10 to 1. The draft with boards and rudders up is 1.4 foot. Boards down maybe around 5 foot.

The accommodation is basically 4 berths in the hulls with 5.1 foot headroom and a toilet. The pod main cabin has a dinette that converts to a double berth, a galley and a navigation area. The headroom is 5.1 foot with the roof hatch closed or 6.5 foot with the hatch open.

Now we come to the construction. The standard construction is 9 mm ply on the hulls and bulkheads with stringers and supporting timber on the frames. The majority of the rest of the structure is 9 mm with some 6 mm on the decks and in the pod cabin. Main beam and aft beam 9 mm ply 75 x 50 mm timber top and bottom flange 9 mm ply 75 x 50 mm timber top and bottom flange 9 mm ply. Box section 170 x 220 mm (see jpeg).

Now I know that Guzzi3 is a fan of foam glass and I have not seen Saturn plans to know if a foam glass option is available. But Rob Denney in one thread suggested that a foam glass version of Saturn could be lighter. He suggested 10mm foam with 600 gsm glass outside and 400 gsm inside infused with vinylester or epoxy resin. The infused resin would be 1:2 resin to fibre ratio. Rob was trying to demonstrate that if you replaced 9 mm ply on a design with foam glass you could save weight, Rob was NOT trying to redesign Richards work. How much weight could be saved? Rob estimated 1100 lbs. That is a very large weight saving and would allow a payload of 2,500 lbs not the standard Saturn payload of 1,400 lbs. Please speak to Richard Woods if you want to do a foam glass Saturn.

The performance of a Saturn is good. A Canadian owner said “The last leg of the sail from Nanaimo was in South East 15- 20 knots and the typical Georgia strait chop that Tucanu (24 foot Woods cat) sometimes struggled in. We flew along averaging 9 knots and again made a 14 knots high. She is so easy to handle in these difficult conditions and again was astonishingly dry and comfortable. The great thing about surfing down those 4ft swells is that the forward chines give great dynamic lift to keep you moving and in control.” Also “The day after Boxing Day I took my kids and their partners out for a windy sail in relatively flat water. With a reef in and a few rolls in the jib I guess we got up to 17kts or more very easily going to windward with no fuss at all”. Richard estimated that the Saturn could keep pace with a F24 and it has proven so in practice.

The jpegs give an idea but the following website and Zip files (3 downloads in the middle of the page) give a lot more photos etc. Sailing Catamarans - Saturn - 8.8m hard chine performance cruiser with central cuddy http://www.sailingcatamarans.com/index.php/designs/3-25ft-to-30ft-catamarans-designs/173-saturn-88m-hard-chine-performance-cruiser-with-central-cuddy-p600

 

Cardboard boats that can carry a crew can be useful. If you need to prototype an idea, produce a cheap one off for some kids to play with very close to shore for a couple of hours or become “World Champion” in cardboard canoe racing etc. The images below give the idea of what some of the cardboard boats that can be built. The proa carrying 4 paddlers is the “world champion” cardboard boat from Arkansas and the large 4 person tri was a racing boat built by the Mythbuster TV team. There is even a small cardboard power boat in one jpeg.

Most of these boats use cardboard from packaging and duct tape as there joining mechanism. The surface is sealed with house paint or glues and some newspaper. But if you want a serious boat you can use thicker corrugated cardboard or fluted cardboard from large produce bins that can hold up to 1000 lbs of EG watermelons or pumpkins from bulk fruit and vegetable stores. In the design process of a cardboard boat you need to decide how long you want to keep it. If you plan to have it for a few weeks for experimentation you need Rigidity longitudinally and to reinforce the skin. Rigidity can be obtained by either cardboard ribs or thick cardboard used to help make the boats rigid enough to support the weight of humans in the vessels. Some thicknesses of multilevel of cardboard can be up to 25 mm thick. Cross beam tubes can be thick cardboard rolls used in carpet or vinyl transport (big box hardwood stores) others have used PVC pipes.

The primary driver of longevity is “waterproofing” that can range from EG several layers of exterior house paint to a light fiberglass skin (some cardboard racing rules do not allow resin or fiberglass). But remember these are cheap disposable boats. Build cheap but smart and they will be useful for a few months of fun, you will get to try ideas and learn a lot about design etc. But the primary rule in all cardboard boats is wear a LIFEJACKET.

The jpegs give the idea. There is also a 4 meg PDF giving detail of how to design and build a cardboard boat. Have fun.

 

Mr Woods has given me layups for foam. 12mm 80kg/m^3 and 600gsm either side. Rob suggests lighter. I probably don't need more payload and the $ are small.

Perception always fascinates me. Both Gypsy and Saturn have the same claimed payload of 600kg yet gypsy is billed an ocean crosser and saturn and racer/ coastal cruiser.

Mr Woods has told me increasing the hull height 4" in fine. That would give me standing headroom in the hulls as I am 5'6". 6' is a nice idea and if you are tall...maybe Gypsy would be a better option.

As I mentioned previously Gypsy can be built round bottom or dory. Saturn offers the middle path multi chine. That's why I thought it of interest. We are spoiled for choice...

 

600gm is ok for topsides but below waterline could use a bit more, depends on intended use really. Going lighter on inside skin is pretty standard, especially if using a tensile fabric like Kevlar.
An alternative to more glass below waterline is denser foam.